Understanding Bt Hybrids and Their IRM Programs

Table 1. Characteristics of currently available Bt field corn. Bt Event and/or Cry-Protein Registrant Company Trade Name Efficacy Bt-11, Cry1Ab Syngenta Seeds, Inc. YieldGard, Agrisure Control of European corn borer, southwestern corn borer and southern cornstalk borer. Suppression of corn earworm, fall armyworm and common stalk borer. MIR604, mCry3A Syngenta Seeds, Inc. Agrisure RW Control of corn rootworm Bt-11, Cry1Ab + MIR604, mCry3A Syngenta Seeds, Inc. Agrisure CB/RW Control of corn rootworm and European corn borer (see Bt 11 above) TC1507, Cry1F Mycogen Seeds c/o Dow Agrosciences LLC; Pioneer Hi-Bred International Inc./DuPont Herculex I Control of European corn borer, southwestern corn borer, western bean cutworm, fall armyworm and black cutworm. Suppression of corn earworm. DAS-59122-7, Cry34/35Ab1 Pioneer Hi-Bred /Dupont and Mycogen Seeds/ DowAgroSciences Herculex RW Corn Rootworm Cry1F + Cry34/35Ab1 (Bt corn stack) PioneerHi-Bred /Dupont and Mycogen Seeds/ DowAgroSciences Herculex Xtra European corn borer (see TC1507) plus corn rootworm MON810, Cry1Ab Monsanto Co. YieldGard Control of European corn borer, southwestern corn borer and southern cornstalk borer. Suppression of corn earworm, fall armyworm and common stalk borer. MON863, Cry3Bb1 Monsanto YieldGard Rootworm Corn rootworm Cry1Ab + Cry3Bb1 (Bt corn stack) Monsanto YieldGard Plus European corn borer (see Mon810) plus corn rootworm MON88017/Cry3Bb1 + Mon810, Cry1Ab Monsanto YieldGard VT Triple European corn borer (see Mon810) plus corn rootworm MON88017/Cry3Bb1 Monsanto YieldGard VT Rootworm Corn rootworm Under Development (Not Commercially Available)  MON89034/Cry 1A.105 +Cry 2Ab2 Monsanto YieldGard VT Pro European corn borer, fall armyworm, black cutworm, and corn earworm MON88017/Cry3Bb + MON89034/Cry 1A.105 + Cry 2Ab2 Monsanto YieldGard VT Triple Pro Corn rootworm and European corn borer, fall armyworm, black cutworm, and corn earworm

Bt Corn Hybrids

First, what is an "event" with respect to Bt corn? An event is the successful insertion of a gene encoding a Bt protein into corn plant DNA. Events vary in the type of Bt protein produced, the expression of the protein and the target insect pest. Expression refers to where, how much, and at what plant stages the Bt protein is produced in the corn plant.

The following events will be in a variety of corn hybrids and offered through several seed companies. Some have been around for several years (e.g. YieldGard) and some are relatively new (e.g. Herculex RW). Note that the events "suppress" some other insects. If these other pests are your primary concern, the Bt hybrids may not satisfactorily control them.

Up to date market approval status of biotech corn hybrids can be obtained at the National Corn Growers Web Site http://www.ncga.com/. Look under “Know Before You Grow” to learn if specific biotech corn hybrids are approved for export.

Resistance Management

Following are the two basic IRM programs for Bt rootworm corn (A) and Bt European corn borer corn (B). If you are growing a Bt corn stack that has genes for both corn borer and rootworm (YieldGard Plus, Herculex Xtra, Agrisure RW/CB), you must follow the IRM for rootworm program (A).

A. IRM for Bt corn hybrids active against corn rootworm

1. Growers must plant a structured refuge of at least 20% non-Bt rootworm corn that may be treated with insecticides (e.g., seed treatments, liquid or granular) as needed to control corn rootworm larvae.
2. Growers will not be permitted to apply corn rootworm labeled insecticides to the refuge for control of insect pests (either rootworm beetles or other insects, e.g., western bean cutworms, grasshoppers, etc.) while adult corn rootworm are present unless the Bt rootworm field is treated in a similar manner.
3. Refuge plantings may be blocks in or adjacent to Bt rootworm cornfields or as in-field strips.
4. External refuges must be planted adjacent to Bt rootworm fields.
5. When planting the refuge in strips across a field, refuges must be at least six rows wide for YieldGard Rootworm and YieldGard Plus or at least four rows wide for Herculex RW and Herculex Xtra. It is preferable to have wider strips, such as 12 consecutive row-wide strips.
6. Insecticides for control of corn rootworm larvae may be applied to the refuge area. In addition, the refuge must be planted in similar ground as the Bt corn for rootworm, that is, if the Bt corn for rootworm is planted in ground that was in corn the previous year, so must the refuge. General management of the Bt corn for rootworm and the refuge should be similar, e.g., planting dates, irrigation, fertility programs, etc.

B. IRM for Bt corn hybrids active against European corn borer

1. On each farm, growers may plant up to 80% of their corn acres with Bt corn. At least 20% of their corn acres must be planted with non-corn borer Bt corn and treated only as needed with insecticides. Decisions to treat the refuge must be based on economic thresholds. Conventional Bt products (liquids or granules) must not be used on the non-Bt refuge.
2. Plant non-Bt corn refuge within, adjacent to, or near the Bt cornfields. The refuge must be placed within 1/2 mile of the Bt field, preferably within 1/4 mile.
3. If refuge is established as strips in a field, the strips should be no narrower than four rows. General management of the Bt corn for European corn borer and the refuge should be similar, e.g., planting dates, irrigation, fertility programs, etc.

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